1. Field of the Invention
The present invention relates to fiber optic gyroscopes and more particularly, to a fiber optic gyroscope which employs a high power laser diode as a source.
2. Description of the Prior Art
A number of fiber optic gyroscopes are well known in the prior art. Examples may be found in copending application Ser. No. 08/254,804 filed Jun. 6, 1994 in the name of Avery A. Morgan and assigned to the assignee of the present invention, in U.S. Pat. No. 4,705,399 issued Nov. 10, 1987 and in a paper entitled "INTEGRATED OPTICS: A PRACTICAL SOLUTION FOR THE FIBER-OPTIC GYROSCOPE" BY H. C. Lefevre, S. Vatoux, M. Papuchon and C. Puech presented to the 10th Anniversary Conference 1986 of SPIE and printed in SPIE Vol. 719 Fiber Optic Gyros on pages 101-112. In such systems a fiber optic coil receives light at both ends simultaneously to cause counter propagating beams therein. Under conditions where there is no rotation about an axis normal to the plane of the coil, the beams take the same amount of time to traverse the coil length in both directions and they emerge from the ends in phase. However, if the loop experiences a turning motion about the axis, one beam travels slightly further than the other to complete the loop and emerges displaced in phase. The light from the two ends is passed through a beam splitter to a light detector and because the two beams are displaced in phase, they produce an interference pattern and the intensity measured by the detector is then indicative of the amount of interference and thus indicative of the rate of rotation that the fiber optic loop experiences. Rather than read the output of the detector directly, a closed loop re balance system may be utilized wherein a re balance voltage which increases in a ramp fashion is presented to a modulator located at one end of the fiber optic coil. The modulator operates to introduce an equal but opposite effect on the counter rotating beams so that, at balance, the beams are again in phase and the output of the detector is nulled, or peaked. More particularly, as the ramp voltage increases, the path length is changed a first amount for the beam just entering the fiber at the modulator end but the beam exiting on the other end had been subjected to a different path length change due to the voltage from the modulator which existed when it entered the modulator end. By changing the slope of the ramp the difference between the voltages applied to the entrance and exit beams can be made greater or smaller with the result that the phase shift induced by the modulator effect can be made equal to the phase shift resulting from the rotation.
Prior art fiber optic gyros have utilized relatively low power laser diode sources because high power sources were not economically feasible. Broad band outputs are desirable for gyro use and high power ones (those above 1 milliwatt) have narrow bandwidths. It is, however, desirable in some cases to use high power sources, for example, in order to lower the random noise performance and in cases where one source is needed to power multiple gyros. In the past, Edge Light Emitting Diodes (ELEDs) and Super Luminescent Diodes (SLDs) and certain high power Fiber Light Sources have been considered but at the present time, the ELEDs and the SLDs have not had significantly higher "pigtailed" power (that power available to the gyro) and are rather expensive and the Fiber Light Sources, although having sufficient power, are almost prohibitively expensive. In an article entitled "SCALE-FACTOR STABILIZED FIBER OPTIC GYROSCOPE BASED ON A SPECTRUM BROADENED LASER DIODE SOURCE" by Pie-Yau Chien and Chi-Ling Pan and printed in Vol. 16, No. 6 /Mar. 15, 1991 issue of OPTICS LETTERS, the use of a single mode laser with a broadened spectrum for use in a fiber optic gyro is discussed but such a source would not have multimodes and the light reflection is from the entrance end of the fiber and is thus too close to the source. Another problem encountered in the prior art is mode partition noise which exists in multimode laser diodes. This is a result of the fact that in a multimode source, the diodes have several wavelengths and the output to the gyro jumps between the various wavelengths or modes or parts of various modes. The wavelengths changes is mode partition noise. When a depolarizer is used in the fiber gyro circuit, the mode partition noise is converted to intensity noise which is a more serious problem. With a depolarizer, the amount of light reaching the detector depends on the wavelength from the source and, as the wavelength changes, so does the magnitude of the light reaching the detector. This effect will be further described below.